Where shafts or other components of machinery rotate, one usually finds antifriction bearings. These bearings minimize friction by interposing small rolling elements between the rotatable machine component and the stationary component in which or on which the rotatable component revolves. Typically, the rolling elements move along raceways that are on inner and outer races with one fitted to the rotatable component and the other to the stationary component. The rolling elements may take the form of simple balls, cylindrical rollers, tapered rollers or so-called spherical rollers.
The raceways and the rolling elements remain obscured by the outer race and often an antifriction bearing lies deeply within a machine. Moreover, seals often close the ends of antifriction bearing. Thus, one does not easily inspect an antifriction bearing to determine conditions within its interior.
Apart from conditions that might reveal themselves with disassembly of a bearing, other conditions under which a bearing operates are not discernible from visual inspections. For example, one cannot by visual observations determine the load applied to a bearing or the torque transmitted through the shaft or other component on which the bearing is mounted. Likewise, the temperature at which a bearing operates does not reveal itself to visual observations, but an elevation in temperature certainly signals the onset of problems, and an increase in temperature owing to a lack of lubrication will first manifest itself along the raceways and other critical surfaces.
The physical condition of an antifriction bearing is best determined from within the interior of the bearing itself and the same holds true with regard to the conditions under which a bearing operates. However, extracting information about such conditions has proven to be difficult. Not only are the interiors of antifriction bearings generally inaccessible, but also, the rolling elements, which revolve within them, interfere with instrumentation designed to monitor the interiors of such bearings.